error[E0277]: the trait bound `for<'de> Number: Deserialize<'de>` is not satisfied
 --> $DIR/non_serializable_return_value.rs:5:64
  |
5 |     async fn subtract(&self, subtrahend: i64, minuend: i64) -> Number;
  |                                                                ^^^^^^ the trait `for<'de> Deserialize<'de>` is not implemented for `Number`
  |
  = note: required because of the requirements on the impl of `DeserializeOwned` for `Number`